Age-related changes in muscle strength are an important contributor to frailty in the elderly. The time course and causes for these changes are poorly understood. Our research has three main components. 1) Characterization of longitudinal strength changes in the BLSA. 2) Comparison of exercise response to resistive strength training in young and old subjects through contract with the University of Maryland, College Park. 3) Examination of the motor unit and its relationship to muscle strength and exercise response. For both women and men concentric isokinetic strength was lower in older individuals, declining linearly from the 20s particularly in knee extensors However, women only showed small differences in eccentric strength with increasing age. In men, the degree of change was similar to concentric strength with increasing age. Analysis of longitudinal isometric arm strength and power, showed that power and strength decline in a similar manner with increasing age, though power declines at a faster rate. After accounting for muscle mass, both age and gender have independent effects on both strength and power. The exercise training study is underway, with over 30 subjects having begun the research project. A protocol was developed to examine motor unit function at different levels of muscle exertion in the quadriceps which is being employed in the exercise contract, and will be implemented in the BLSA. A strong positive relationship occurred between the average motor unit size and the force generated. The changes are consistent with the Henneman principle i.e. the order of motor units activation during muscle contractio is from smallest to largest units.